Target for laser produced plasma extreme ultraviolet light source

ABSTRACT

Techniques for generating EUV light include directing a first pulse of radiation toward a target material droplet to form a modified droplet, the first pulse of radiation having an energy sufficient to alter a shape of the target material droplet; directing a second pulse of radiation toward the modified droplet to form an absorption material, the second pulse of radiation having an energy sufficient to change a property of the modified droplet, the property being related to absorption of radiation; and directing an amplified light beam toward the absorption material, the amplified light beam having an energy sufficient to convert at least a portion of the absorption material into extreme ultraviolet (EUV) light.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/152,881, filed Jan. 10, 2014, now allowed, and titled TARGET FORLASER PRODUCED PLASMA EXTREME ULTRAVIOLET LIGHT SOURCE, which is acontinuation of U.S. patent application Ser. No. 13/830,461, filed Mar.14, 2013, now issued, and titled TARGET FOR LASER PRODUCED PLASMAEXTREME ULTRAVIOLET LIGHT SOURCE, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The disclosed subject matter relates to a target for a laser producedplasma extreme ultraviolet light source.

BACKGROUND

Extreme ultraviolet (“EUV”) light, for example, electromagneticradiation having wavelengths of around 50 nm or less (also sometimesreferred to as soft x-rays), and including light at a wavelength ofabout 13 nm, can be used in photolithography processes to produceextremely small features in substrates, for example, silicon wafers.

Methods to produce EUV light include, but are not necessarily limitedto, converting a material that has an element, for example, xenon,lithium, or tin, with an emission line in the EUV range in a plasmastate. In one such method, often termed laser produced plasma (“LPP”),the required plasma can be produced by irradiating a target material,for example, in the form of a droplet, plate, tape, stream, or clusterof material, with an amplified light beam that can be referred to as adrive laser. For this process, the plasma is typically produced in asealed vessel, for example, a vacuum chamber, and monitored usingvarious types of metrology equipment.

SUMMARY

In one general aspect, a method of generating EUV light includesdirecting a first pulse of radiation toward a target material droplet toform a modified droplet, the first pulse of radiation having an energysufficient to alter a shape of the target material droplet; directing asecond pulse of radiation toward the modified droplet to form anabsorption material, the second pulse of radiation having an energysufficient to change a property of the modified droplet, the propertybeing related to absorption of radiation; and directing an amplifiedlight beam toward the absorption material, the amplified light beamhaving an energy sufficient to convert at least a portion of theabsorption material into extreme ultraviolet (EUV) light.

Implementations can include one or more of the following features. Themodified droplet can include a continuous segment of the target materialthat has a width extending along a first direction, and a thicknessextending along a second direction that is different from the firstdirection. The second direction is in the direction of propagation ofthe second pulse of radiation, and the width is greater than thethickness. A plane that includes the first direction can be angledrelative to the direction of propagation of the second pulse ofradiation. The absorption material can include a continuous segment ofthe target material.

The property of the modified droplet can be one or more of an electrondensity and an ion density, and the absorption material can includeplasma adjacent to a surface of a continuous segment of the targetmaterial. The property of the modified droplet can be a surface area.The absorption material can include multiple pieces of the targetmaterial, the multiple pieces having a collective surface area that islarger than the modified droplet.

The first pulse of radiation can be a pulse of light having a wavelengthof 10 μm, a pulse duration of 40 ns, and an energy of 20 mJ, and thesecond pulse of radiation can be a pulse of light wavelength of 1 μm, apulse duration of 10 ns, and an energy of 5 mJ.

The first pulse of radiation can be a pulse of light having a wavelengthof 10 μm, a pulse duration of 20-70 ns, and an energy of 15-60 mJ, andthe second pulse of radiation can be a pulse of light wavelength of 1-10μm, a pulse duration of 10 ns, and an energy of 1-10 mJ.

The first pulse of radiation can be a pulse of light having a wavelengthof 1-10 μm, a pulse duration of 40 ns, and an energy of 20 mJ, and thesecond pulse of radiation can be a pulse of light having a wavelength of1 μm, a pulse duration of 10 ns, and an energy of 1 mJ.

The first pulse of radiation and the second pulse of radiation can bepulses of light having a duration of 1 ns or greater.

The second pulse of radiation can be a pulse of light having a durationof 1 ns to 100 ns.

The second pulse of radiation can be directed toward the modifieddroplet 1-3 μs after the first pulse of radiation is directed toward thetarget material droplet.

The first pulse of radiation can be a pulse of light having a durationof at least 1 ns, and the second pulse of radiation can be a pulse oflight having a duration of at least 1 ns.

In some implementations, at least 2% of the amplified light beam can beconverted to EUV radiation.

The amplified light beam can be a pulse of light, and a subsequent pulseof light can be directed toward a second absorption material no morethan 25 μs after the amplified light beam is directed toward theabsorption material. The second absorption material is formed after theabsorption material and is formed from a second target material droplet.

The first pulse of radiation can be a pulse of radiation having aduration of 300 ps or less. The first pulse of radiation can be a pulseof radiation having a duration of 100 ps-300 ps. The modified dropletcan be a hemisphere shaped volume of particles of target material.

In another general aspect, an extreme ultraviolet light source includesa source that produces an amplified light beam, a first pulse ofradiation, and a second pulse of radiation; a target material deliverysystem; a vacuum chamber coupled to the target material delivery system;and a steering system configured to steer and focus the amplified lightbeam, the first pulse of radiation, and the second pulse of radiationtoward a target location that receives target material from the targetmaterial delivery system in the vacuum chamber. The first pulse ofradiation has an energy sufficient to alter a shape of the targetmaterial droplet to create a modified droplet, the second pulse ofradiation has an energy sufficient to change a property of the modifieddroplet that is related to absorption of radiation, and the amplifiedlight beam is sufficient to convert at least a portion of the absorptionmaterial into extreme ultraviolet (EUV) light.

Implementations can include one or more of the following features. Thesource can include first, second, and third sources, with the firstsource generating the first pulse of radiation, the second sourcegenerating the second pulse of radiation, and the third sourcegenerating the amplified light beam.

The source can include a first source that generates the amplified lightbeam and the first pulse of radiation and a second source that generatesthe second pulse of radiation.

The first source can include a CO₂ laser, and the amplified light beamand the first pulse of radiation can have different wavelengths.

Implementations of any of the techniques described above may include atarget for a laser produced plasma EUV light source, an EUV lightsource, a system for retrofitting an EUV light source, a method, aprocess, a device, executable instructions stored on a computer readablemedium, or an apparatus. The details of one or more implementations areset forth in the accompanying drawings and the description below. Otherfeatures will be apparent from the description and drawings, and fromthe claims.

DRAWING DESCRIPTION

FIG. 1A shows a target material droplet that is converted to anexemplary target.

FIG. 1B is a plot of an exemplary waveform for generating the target ofFIG. 1A.

FIG. 1C shows side views of two pulses of radiation striking targetmaterial.

FIG. 1D is a plot of an exemplary waveform for an EUV light source.

FIG. 2A is a block diagram of a laser produced plasma extremeultraviolet light source.

FIG. 2B is a block diagram of an example of a drive laser system thatcan be used in the light source of FIG. 2A.

FIG. 3A is a top plan view of another laser produced plasma extremeultraviolet (EUV) light source and a lithography tool coupled to the EUVlight source.

FIGS. 3B-3D are top views of a vacuum chamber of the EUV light source ofFIG. 3A at three different times.

FIG. 4 is a flow chart of an exemplary process for producing EUV light.

FIG. 5 is a plot of another exemplary waveform for generating EUV light.

FIGS. 6A-6E are side views of a target material droplet transforminginto a target through interactions with the waveform of FIG. 5.

FIGS. 6F and 6G are side views of the intermediate target of FIG. 6C.

FIG. 7 is a plot of another exemplary waveform for generating EUV light.

FIGS. 8A-8E are side views of a target material droplet transforminginto a target through interactions with the waveform of FIG. 7.

FIG. 9 is a plot of another exemplary waveform for generating EUV light.

FIGS. 10A-10E are side views of a target material droplet transforminginto a target through interactions with the waveform of FIG. 9.

DESCRIPTION

Techniques for producing a target for use in a laser produced plasma(LPP) extreme ultraviolet (EUV) light source are disclosed. The targetis produced by irradiating a target material with two pulses of light insuccession. The first pulse generates an intermediate target and thesecond pulse interacts with the intermediate target to produce thetarget. The target is then irradiated with an amplified light beamhaving energy that is sufficient to convert target material in thetarget to a plasma that emits EUV light. In some implementations, eachof the two pulses of light has a temporal duration or pulse width of atleast 1 nanosecond (ns).

Referring to FIGS. 1A and 1B, an exemplary waveform 5 transforms atarget material 50 into a target 55. The target 55 includes targetmaterial that emits EUV light 57 when converted to plasma. The targetmaterial 50 can be a target mixture that includes a target substance andimpurities such as non-target particles. The target substance is thesubstance that is converted to a plasma state that has an emission linein the EUV range. The target substance can be, for example, a droplet ofliquid or molten metal, a portion of a liquid stream, solid particles orclusters, solid particles contained within liquid droplets, a foam oftarget material, or solid particles contained within a portion of aliquid stream. The target substance, can be, for example, water, tin,lithium, xenon, or any material that, when converted to a plasma state,has an emission line in the EUV range. For example, the target substancecan be the element tin, which can be used as pure tin (Sn); as a tincompound, for example, SnBr₄, SnBr₂, SnH₄; as a tin alloy, for example,tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or anycombination of these alloys. Moreover, in the situation in which thereare no impurities, the target material includes only the targetsubstance. The discussion below provides an example in which the targetmaterial 50 is a target material droplet made of molten metal. Thetarget material 50 is referred to as the target material droplet 50.However, the target material 50 can take other forms.

FIG. 1A shows the target material droplet 50 physically transforminginto an intermediate target 51 and then into the target 55 over a timeperiod. The target material droplet 50 is transformed throughinteraction with the radiation delivered in time according to thewaveform 5. FIG. 1B is a plot of the energy in the waveform 5 as afunction of time over the time period of FIG. 1A. As compared to thetarget material droplet 50 and the intermediate target 51, the target 55absorbs more of an amplified light beam 8 and converts a larger portionof the energy in the amplified light beam 8 to the EUV light 57.

The waveform 5 is a representation of the energy that interacts with thetarget material droplet 50 and its modified forms over time. Althoughthe waveform 5 is shown as a single waveform as a function of time,various portions of the waveform 5 can be produced by different sources.The waveform 5 includes a representation of a first pulse of radiation 6(a first pre-pulse 6) and a representation of a second pulse ofradiation 7 (a second pre-pulse 7). The first pre-pulse 6 and the secondpre-pulse 7 can be any type of pulsed radiation that has sufficientenergy to act on the target material droplet 50 and the intermediatetarget 51, respectively. Examples of pre-pulses are discussed withrespect to FIGS. 3A-3D, 4, 5, 7 and 9.

The first pre-pulse 6 occurs at a time t=t₁ and has a pulse duration 12,and the second pre-pulse 7 occurs at a time t=t₂ and has a pulseduration 14. The pulse duration can be represented by the full width athalf maximum, the amount of time that the pulse has an intensity that isat least half of the maximum intensity of the pulse. However, othermetrics can be used to determine the pulse duration. The times t₁ and t₂are separated by a first delay time 11, with the second pre-pulse 7occurring after the first pre-pulse 6.

The waveform 5 also shows a representation of the amplified light beam8. The amplified light beam 8 can be referred to as the main beam or themain pulse. The amplified light beam 8 has sufficient energy to converttarget material in the target 55 to plasma that emits EUV light. Thesecond pre-pulse 7 and the amplified light beam 8 are separated in timeby a second delay time 13, with the amplified light beam 8 occurringafter the second pre-pulse 7.

Examples of an EUV light source that can produce and/or use the target55 are shown in FIGS. 2A, 2B, and 3A-3E. Before discussing the EUV lightsources, a discussion of the interactions of the pulses of light,including the first pre-pulse 6 and the second pre-pulse 7, with thetarget material droplet 50 and the intermediate target 51 is provided.

When a laser pulse impinges (strikes) a target material droplet that ismetallic, the leading edge of the pulse sees (interacts with) a surfacethat is a reflective metal. The target material droplet 50 reflects mostof the energy in the leading edge of the pulse and absorbs little. Thesmall amount that is absorbed heats the surface of the droplet,evaporating and ablating the surface. The target material that isevaporated from the surface of the droplet forms a cloud of electronsand ions close to the surface. As the pulse of radiation continues toimpinge on the target material droplet, the electric field of the laserpulse can cause the electrons in the cloud to move. The moving electronscollide with nearby ions, heating the ions through the transfer ofkinetic energy at a rate that is roughly proportional to the product ofthe densities of the electrons and the ions in the cloud. Through thecombination of the moving electrons striking the ions and the heating ofthe ions, the cloud absorbs the pulse.

As the cloud is exposed to the later parts of the laser pulse, theelectrons in the cloud continue to move and collide with ions, and theions in the cloud continue to heat. The electrons spread out andtransfer heat to the surface of the target material droplet (or bulkmaterial that underlies the cloud), further evaporating the surface ofthe target material droplet. The electron density in the cloud increasesin the portion of the cloud that is closest to the surface of the targetmaterial droplet. The cloud can reach a point where the density ofelectrons increases such that the portions of the cloud reflect thelaser pulse instead of absorbing it.

The present technique for generating a target for an LPP EUV lightsource applies two pre-pulses to a target material droplet to physicallytransform the target material droplet into a target that more readilyabsorbs energy. The first pre-pulse 6 forms a geometric distribution oftarget material that becomes the intermediate target 51. The secondpre-pulse 7 transforms the intermediate target 51 into the target 55.The first pre-pulse 6 and the second pre-pulse 7 are discussed in turnbelow.

Referring also to FIG. 1C, the first pre-pulse 6 physically transformsthe target material droplet into a geometric distribution 52 of targetmaterial. The geometric distribution 52 can be a material that is notionized (a material that is not a plasma). The geometric distribution 52can be, for example, a disk of liquid or molten metal, a continuoussegment of target material that does not have voids or substantial gaps,a mist of micro- or nano-particles, or a cloud of atomic vapor. Thegeometric distribution 52 expands spatially during the first delay time11 and becomes the intermediate target 51. The first pre-pulse 6 spreadsthe target material droplet 50 spatially. Spreading the target materialdroplet 50 can have two effects.

First, the intermediate target 51 generated by the first pre-pulse 6 hasa form that presents a larger area to an oncoming pulse of radiation(such as the pre-pulse 7). The intermediate target 51 has across-sectional diameter 54 that is larger than a beam diameter 57 ofthe pre-pulse 7 such that the intermediate target receives the entirepre-pulse 7. Additionally, the intermediate target 51 can have athickness 58 that is thinner in a direction of propagation of thepre-pulse 7 than a thickness 59 of the target material droplet 50. Therelative thinness of the intermediate target 51 allows the pre-pulsebeam 7 to irradiate more of the target material that is in theintermediate target 51, including more of the target material that isnot irradiated by the pre-pulse 7 when it initially reaches theintermediate target 51.

Second, spreading the target material of the droplet 50 out spatiallycan minimize the occurrence of regions of excessively high materialdensity during heating of the plasma by the strong pulse 8, which canblock generated EUV light. If the plasma density is high throughout aregion that is irradiated with a laser pulse, absorption of the laserpulse is limited to the portions of the region that receives the laserpulse first. Heat generated by this absorption may be too distant fromthe bulk target material to maintain the process of evaporating andheating of the target material surface long enough to utilize(evaporate) a meaningful amount of the bulk target material during thefinite duration of the pulse 8. In instances where the region has a highelectron density, the light pulse only penetrates a fraction of the wayinto the region before reaching a “critical surface” where the electrondensity is so high that the light pulse is reflected. The light pulsecannot travel into those portions of the region and little EUV light isgenerated from target material in those regions. The region of highplasma density can also block EUV light that is emitted from theportions of the region that do emit EUV light. Consequently, the totalamount of EUV light that is emitted from the region is less than itwould be if the region lacked the portions of high plasma density. Assuch, spreading the target material droplet 10 into the larger volume ofthe intermediate target 51 means that an incident light beam reachesmore of the material in the intermediate target 51 before beingreflected. This can increase the amount of EUV light subsequentlyproduced.

The waveform 5 also shows a representation of the second pre-pulse 7.The second pre-pulse 7 impinges on the intermediate target 51 and formsthe target 55 before the amplified light beam 8 arrives. The target 55can take many forms. For example, the target 55 can be a pre-plasma thatis spatially near to a bulk target material. A pre-plasma is a plasmathat is used to enhance absorption of incident light (such as thepre-pulse 7 or the amplified light beam). Although the pre-plasma canemit small amounts of EUV light in some instances, the EUV light that isemitted is not of the wavelength or amount that is emitted by the target55. In other implementations, the target 55 is a volume of fragments ora mist of target material. An example of a waveform that includes asecond pre-pulse that can form a pre-plasma is discussed below withrespect to FIG. 5. An example of a waveform that includes a secondpre-pulse that can form fragments of target material is discussed belowwith respect to FIG. 7. In yet other implementations, the target 55 is apre-plasma formed close to a collection of particles of target materialdistributed throughout a hemisphere shaped volume. An example of such atarget is discussed below with respect to FIG. 9.

In some implementations, the pulse duration 12 of the first pre-pulse 6and the pulse duration 14 of the second pre-pulse 7 are 1 ns or greater.Using two pre-pulses that are greater than 1 ns allows the target 55 tobe produced using pulses of radiation that are generated without using alaser that generates picosecond (ps) or shorter pulses. Lasers that emitns-duration pulses and have relatively high repetition rates (50 kHz-100kHz) can be more readily available than those that emit ps-pulses. Useof higher-repetition rate ns-pulse generating lasers to generate thepre-pulses 6 and 7 allows an EUV light source that uses the target 55 tohave a higher overall system repetition rate.

FIG. 1D shows an exemplary plot of a waveform 60 over two continuouscycles of an EUV light source. The waveform 60 is two instances of thewaveform 5 (FIG. 1A), with each cycle of the EUV light source applyingan instance of the waveform 5 to two separate target material droplets(one per cycle) to emit EUV light once per cycle. In the example shownin FIG. 1D, EUV light emissions 61 and 62 occur after an instance of thewaveform 5 is applied to a target material droplet. The emissions 61 and62 are separated in time by a time 64 that is the inverse of therepetition rate of the EUV light source. The repetition rate of the EUVlight source also can be considered as the minimum amount of timebetween two successive EUV light emissions. Because the time between theEUV light emissions 61 and 62 depends on how quickly instances of thewaveform 5 can be generated, the repetition rate of the sources thatgenerate the pre-pulses 6 and 7 at least partially determines the systemrepetition rate. When using two ns-duration pulses as the pre-pulses 6and 7, the EUV light source's system repetition rate can be, forexample, 40 kHz-100 kHz.

Although the example of FIG. 1D shows continuous emission of EUV light,where EUV light is emitted at periodic intervals determined by thesystem repetition rate, the EUV light source can be operated in othermodes depending on the needs of a lithography tool that receives thegenerated EUV light. For example, the EUV light source also can beoperated or set to emit EUV light in bursts that are separated in timeby an amount greater than the system repetition rate or at an irregularinterval. The system repetition rate discussed with respect to FIG. 1Ais provided as an example of a minimum amount of time between EUV lightemissions.

FIGS. 2A, 2B, and 3A-3C show exemplary LPP EUV light sources in whichthe target 55 can be used.

Referring to FIG. 2A, an LPP EUV light source 100 is formed byirradiating a target mixture 114 at a target location 105 with anamplified light beam 110 that travels along a beam path toward thetarget mixture 114. The target location 105, which is also referred toas the irradiation site, is within an interior 107 of a vacuum chamber130. When the amplified light beam 110 strikes the target mixture 114, atarget material within the target mixture 114 is converted into a plasmastate that has an element with an emission line in the EUV range. Thecreated plasma has certain characteristics that depend on thecomposition of the target material within the target mixture 114. Thesecharacteristics can include the wavelength of the EUV light produced bythe plasma and the type and amount of debris released from the plasma.

The light source 100 also includes a target material delivery system 125that delivers, controls, and directs the target mixture 114 in the formof liquid droplets, a liquid stream, solid particles or clusters, solidparticles contained within liquid droplets or solid particles containedwithin a liquid stream. The target mixture 114 can also includeimpurities such as non-target particles. The target mixture 114 isdelivered by the target material delivery system 125 into the interior107 of the chamber 130 and to the target location 105.

The light source 100 includes a drive laser system 115 that produces theamplified light beam 110 due to a population inversion within the gainmedium or mediums of the laser system 115. The light source 100 includesa beam delivery system between the laser system 115 and the targetlocation 105, the beam delivery system including a beam transport system120 and a focus assembly 122. The beam transport system 120 receives theamplified light beam 110 from the laser system 115, and steers andmodifies the amplified light beam 110 as needed and outputs theamplified light beam 110 to the focus assembly 122. The focus assembly122 receives the amplified light beam 110 and focuses the beam 110 tothe target location 105.

In some implementations, the laser system 115 can include one or moreoptical amplifiers, lasers, and/or lamps for providing one or more mainpulses and, in some cases, one or more pre-pulses. Each opticalamplifier includes a gain medium capable of optically amplifying thedesired wavelength at a high gain, an excitation source, and internaloptics. The optical amplifier may or may not have laser mirrors or otherfeedback devices that form a laser cavity. Thus, the laser system 115produces an amplified light beam 110 due to the population inversion inthe gain media of the laser amplifiers even if there is no laser cavity.Moreover, the laser system 115 can produce an amplified light beam 110that is a coherent laser beam if there is a laser cavity to provideenough feedback to the laser system 115. The term “amplified light beam”encompasses one or more of: light from the laser system 115 that ismerely amplified but not necessarily a coherent laser oscillation andlight from the laser system 115 that is amplified (externally or withina gain medium in the oscillator) and is also a coherent laseroscillation.

The optical amplifiers in the laser system 115 can include as a gainmedium a filling gas that includes CO₂ and can amplify light at awavelength of between about 9100 and about 11000 nm, and in particular,at about 10.6 μm, at a gain greater than or equal to 1000. In someexamples, the optical amplifiers amplify light at a wavelength of 10.59μm. Suitable amplifiers and lasers for use in the laser system 115 caninclude a pulsed laser device, for example, a pulsed, gas-discharge CO₂laser device producing radiation at about 9300 nm or about 10600 nm, forexample, with DC or RF excitation, operating at relatively high power,for example, 10 kW or higher and high pulse repetition rate, forexample, 50 kHz or more. The optical amplifiers in the laser system 115can also include a cooling system such as water that can be used whenoperating the laser system 115 at higher powers.

FIG. 2B shows a block diagram of an example drive laser system 180. Thedrive laser system 180 can be used as the drive laser system 115 in thesource 100. The drive laser system 180 includes three power amplifiers181, 182, and 183. Any or all of the power amplifiers 181, 182, and 183can include internal optical elements (not shown). The power amplifiers181, 182, and 183 each include a gain medium in which amplificationoccurs when pumped with an external electrical or optical source.

Light 184 exits from the power amplifier 181 through an output window185 and is reflected off a curved mirror 186. After reflection, thelight 184 passes through a spatial filter 187, is reflected off of acurved mirror 188, and enters the power amplifier 182 through an inputwindow 189. The light 184 is amplified in the power amplifier 182 andredirected out of the power amplifier 182 through an output window 190as light 191. The light 191 is directed toward the amplifier 183 withfold mirrors 192 and enters the amplifier 183 through an input window193. The amplifier 183 amplifies the light 191 and directs the light 191out of the amplifier 183 through an output window 194 as an output beam195. A fold mirror 196 directs the output beam 195 upwards (out of thepage) and toward the beam transport system 120.

The spatial filter 187 defines an aperture 197, which can be, forexample, a circle through which the light 184 passes. The curved mirrors186 and 188 can be, for example, off-axis parabola mirrors with focallengths of about 1.7 m and 2.3 m, respectively. The spatial filter 187can be positioned such that the aperture 197 coincides with a focalpoint of the drive laser system 180. The example of FIG. 2B shows threepower amplifiers. However, more or fewer power amplifiers can be used.

Referring again to FIG. 2A, the light source 100 includes a collectormirror 135 having an aperture 140 to allow the amplified light beam 110to pass through and reach the target location 105. The collector mirror135 can be, for example, an ellipsoidal mirror that has a primary focusat the target location 105 and a secondary focus at an intermediatelocation 145 (also called an intermediate focus) where the EUV light 106can be output from the light source 100 and can be input to, forexample, an integrated circuit beam positioning system tool (not shown).The light source 100 can also include an open-ended, hollow conicalshroud 150 (for example, a gas cone) that tapers toward the targetlocation 105 from the collector mirror 135 to reduce the amount ofplasma-generated debris that enters the focus assembly 122 and/or thebeam transport system 120 while allowing the amplified light beam 110 toreach the target location 105. For this purpose, a gas flow can beprovided in the shroud that is directed toward the target location 105.

The light source 100 can also include a master controller 155 that isconnected to a droplet position detection feedback system 156, a lasercontrol system 157, and a beam control system 158. The light source 100can include one or more target or droplet imagers 160 that provide anoutput indicative of the position of a droplet, for example, relative tothe target location 105 and provide this output to the droplet positiondetection feedback system 156, which can, for example, compute a dropletposition and trajectory from which a droplet position error can becomputed either on a droplet by droplet basis or on average. The dropletposition detection feedback system 156 thus provides the dropletposition error as an input to the master controller 155. The mastercontroller 155 can therefore provide a laser position, direction, andtiming correction signal, for example, to the laser control system 157that can be used, for example, to control the laser timing circuitand/or to the beam control system 158 to control an amplified light beamposition and shaping of the beam transport system 120 to change thelocation and/or focal power of the beam focal spot within the chamber130.

The target material delivery system 125 includes a target materialdelivery control system 126 that is operable in response to a signalfrom the master controller 155, for example, to modify the release pointof the droplets as released by a target material supply apparatus 127 tocorrect for errors in the droplets arriving at the desired targetlocation 105.

Additionally, the light source 100 can include a light source detector165 that measures one or more EUV light parameters, including but notlimited to, pulse energy, energy distribution as a function ofwavelength, energy within a particular band of wavelengths, energyoutside of a particular band of wavelengths, and angular distribution ofEUV intensity and/or average power. The light source detector 165generates a feedback signal for use by the master controller 155. Thefeedback signal can be, for example, indicative of the errors inparameters such as the timing and focus of the laser pulses to properlyintercept the droplets in the right place and time for effective andefficient EUV light production.

The light source 100 can also include a guide laser 175 that can be usedto align various sections of the light source 100 or to assist insteering the amplified light beam 110 to the target location 105. Inconnection with the guide laser 175, the light source 100 includes ametrology system 124 that is placed within the focus assembly 122 tosample a portion of light from the guide laser 175 and the amplifiedlight beam 110. In other implementations, the metrology system 124 isplaced within the beam transport system 120. The metrology system 124can include an optical element that samples or re-directs a subset ofthe light, such optical element being made out of any material that canwithstand the powers of the guide laser beam and the amplified lightbeam 110. A beam analysis system is formed from the metrology system 124and the master controller 155 since the master controller 155 analyzesthe sampled light from the guide laser 175 and uses this information toadjust components within the focus assembly 122 through the beam controlsystem 158.

Thus, in summary, the light source 100 produces an amplified light beam110 that is directed along the beam path to irradiate the target mixture114 at the target location 105 to convert the target material within themixture 114 into plasma that emits light in the EUV range. The amplifiedlight beam 110 operates at a particular wavelength (that is alsoreferred to as a source wavelength) that is determined based on thedesign and properties of the laser system 115. Additionally, theamplified light beam 110 can be a laser beam when the target materialprovides enough feedback back into the laser system 115 to producecoherent laser light or if the drive laser system 115 includes suitableoptical feedback to form a laser cavity.

Referring to FIG. 3A, a top plan view of an exemplary optical imagingsystem 300 is shown. The optical imaging system 300 includes an LPP EUVlight source 305 that provides EUV light to a lithography tool 310. Thelight source 305 can be similar to, and/or include some or all of thecomponents of, the light source 100 of FIGS. 2A and 2B. As discussedbelow, the target 55 can be used in the light source 305 to increase theamount of light emitted by the light source 305.

The light source 305 includes a drive laser system 315, an opticalelement 322, a pre-pulse source 324, a focusing assembly 326, a vacuumchamber 340, and a EUV collecting optic 346. The EUV collecting optic346 directs the EUV light emitted by the target 55 to the lithographytool 310. The EUV collecting optic 346 can be the collector mirror 135of FIG. 2A.

The drive laser system 315 produces an amplified light beam 316. Theamplified light beam 316 can be similar to the amplified light beam 18of FIGS. 1A-1C and can be referred to as a main pulse or a main beam.The amplified light beam 316 has an energy sufficient to convert targetmaterial in the target 55 into plasma that emits EUV light.

The pre-pulse source 324 emits pulses of radiation 317 and 318. Thepulses of radiation 317 and 318 can be similar to the first pre-pulse 6and the second pre-pulse 7 of FIG. 1B. The pre-pulse source 324 can be,for example, a Q-switched Nd:YAG laser that operates at a 50 kHzrepetition rate, and the pulses of radiation 317 and 318 can be pulsesfrom the Nd:YAG laser that have a wavelength of 1.06 μm. The repetitionrate of the pre-pulse source 324 indicates how often the pre-pulsesource 324 produces a pulse of radiation. For the example where thepre-pulse source 324 has a 50 kHz repetition rate, a pulse of radiation317 is emitted from the source 324 every 20 microseconds (μs).

Other sources can be used as the pre-pulse source 324. For example, thepre-pulse source 324 can be any rare-earth-doped solid state laser otherthat an Nd:YAG, such as an erbium-doped fiber (Er:glass) laser. Thepre-pulse source 324 can be any other radiation or light source thatproduces light pulses that have an energy and wavelength used for thefirst pre-pulse 6 and the second pre-pulse 7.

The optical element 322 directs the amplified light beam 316 and thepulses of radiation 317 and 318 from the pre-pulse source 324 to thechamber 340. The optical element 322 is any element that can direct theamplified light beam 316 and the pulses of radiation 317 and 318 alongsimilar paths and deliver the amplified light beam 316 and the pulses ofradiation 317 and 318 to the chamber 340. In the example shown in FIG.3A, the optical element 322 is a dichroic beamsplitter that receives theamplified light beam 316 and reflects it toward the chamber 340. Theoptical element 322 receives the pulses of radiation 317 and 318 andtransmits the pulses toward the optical chamber 340. The dichroicbeamsplitter has a coating that reflects the wavelength(s)s of theamplified light beam 316 and transmits the wavelength(s) of the pulsesof radiation 317 and 318. The dichroic beamsplitter can be made of, forexample, diamond.

In other implementations, the optical element 322 is a mirror thatdefines an aperture (not shown). In this implementation, the amplifiedlight beam 316 is reflected from the mirror surface and directed towardthe chamber 340, and the pulses of radiation pass through the apertureand propagate toward the chamber 340.

In still other implementations, a wedge-shaped optic (for example, aprism) can be used to separate the main pulse 316, the pre-pulse 317,and the pre-pulse 318 into different angles, according to theirwavelengths. The wedge-shaped optic can be used in addition to theoptical element 322, or it can be used as the optical element 322. Thewedge-shaped optic can be positioned just upstream (in the “−z”direction) of the focusing assembly 326.

Additionally, the pulses 317 and 318 can be delivered to the chamber 340in other ways. For example, the pulses 317 and 318 can travel throughoptical fibers that deliver the pulses 317 and 318 to the chamber 340and/or the focusing assembly 326 without the use of the optical element322 or other directing elements. In these implementations, the fibersbring the pulses of radiation 317 and 318 directly to an interior of thechamber 340 through an opening formed in a wall of the chamber 340.

Returning to the example of FIG. 3A, the amplified light beam 316 fromthe drive laser system 315 is reflected from the optical element 322 andpropagates through the focusing assembly 326. The focusing assembly 326focuses the amplified light beam 316 onto the target location 342. Thepulses of radiation 317 and 318 pass through the optical element 322 andare directed through the focusing assembly 326 to the chamber 340.Referring to FIGS. 3B-3D, each of the amplified light beam 316, thepulse of radiation 317, and the pulse of radiation 318 are directed todifferent locations along the “x” direction in the chamber 340.

Referring also to FIGS. 3B-3D, a top view of the target material supplyapparatus 347 releasing a stream of target material droplets in the “x”direction toward the target location 342 is shown. The stream includesdroplets 348 a and 348 b. The target location 342 is a location thatreceives the amplified light beam 316 and also can be at the focal pointof the EUV collecting optic 346. FIG. 3B shows the chamber 340 at a timet=t₁, FIG. 3C shows the chamber 340 at a time t=t₂ that occurs aftert=t₁, and FIG. 3C shows the chamber 340 at a time t=t₃, which occursafter t=t₂.

Each of the amplified light beam 316 and the pulses of radiation 317 and318 are directed toward different locations along the “x” direction inthe chamber 340 at different times. This allows a target materialdroplet to be converted into a target before reaching the targetlocation 342. FIGS. 3B-3D show an example of a target material droplet(the target material droplet 348 a) being converted into the target 55.A time=t₁ (FIG. 3B), the pulsed beam of radiation 317 irradiates thetarget material droplet 348 a at the time “t₁” at location that isdisplaced in the “−x” direction from the target location 342. The pulsedbeam of radiation 317 transforms the target material droplet 348 b intothe intermediate target 51. At the time=t₂ (FIG. 3C), the intermediatetarget 51 has moved in the “x” direction closer to the target location342 and arrives at another location that is displaced in the “−x”direction relative to the target location 342. The pulse beam ofradiation 318 irradiates the intermediate target 51 and transforms itinto the target 55. The target 55 travels in the “x” direction andarrives at the target location 342 without being substantially ionized.In this manner, the target 55 can be a pre-formed target that is formedat a time before the target 55 enters the target location 342. At thetime=t₃ (FIG. 3D), the amplified light beam 316 irradiates the target 55to produce plasma that emits EUV light.

In the example shown in FIG. 3A, a single block represents the pre-pulsesource 324. The pre-pulse source 324 can be a single light source or aplurality of light sources; for example, two separate sources can beused to generate the pulses 317 and 318. The two separate sources can bedifferent types of sources that produce pulses of radiation havingdifferent wavelengths and energies. For example, the pulse 317 can havea wavelength of 10.6 μm and be generated by a CO₂ laser, and the pulse318 can have a wavelength of 1.06 μm and be generated by arare-earth-doped solid state laser.

In some implementations, the pulse of radiation 317 can be generated bythe drive laser system 315. For example, the drive laser system caninclude two CO₂ seed laser subsystems and one amplifier. One of the seedlaser subsystems can produce an amplified light beam having a wavelengthof 10.26 μm, and the other seed laser subsystem can produce an amplifiedlight beam having a wavelength of 10.59 μm. These two wavelengths cancome from different lines of the CO₂ laser. In other examples, otherlines of the CO₂ laser can be used to generate the two amplified lightbeams. Both amplified light beams from the two seed laser subsystems areamplified in the same power amplifier chain and then angularly dispersedto reach different locations within the chamber 340. The amplified lightbeam with the wavelength of 10.26 μm can be used as the pre-pulse 317,and the amplified light beam with the wavelength of 10.59 μm can be usedas the amplified light beam 316.

Moreover, the amplified light beam 316, the pulse of radiation 317, andthe pulse of radiation 318 are all amplified in the same amplifier. Forexample, the three power amplifiers 181, 182, and 183 (FIG. 1B) can beused to amplify all of the amplified light beam 316, the pre-pulse 317,and the pre-pulse 318. In this implementation, the amplifier can havethree seed lasers, one of which is used to generate each of theamplified light beam 316, the pulse of radiation 317, and the pulse ofradiation 318. More or fewer seed lasers can be used.

Referring to FIG. 4, a flow chart of an example process 400 forgenerating EUV light is shown. The process 400 can be performed usingthe light source 100 or the light source 305.

A first pulse of radiation is directed toward a target material dropletto form an altered droplet (410). The first pulse of radiation can be apulse that has an energy that is sufficient to alter a shape of thetarget material droplet. The first pulse of radiation can have aduration of at least 1 ns, for example, the first pulse of radiation canhave a duration of 1-100 ns and a wavelength of 1 μm or 10 μm. In oneexample, the first pulse of radiation can be a laser pulse that hasenergy of 15-60 mJ, a pulse duration of 20-70 ns, and a wavelength of1-10 μm. In some examples, the first pulse of radiation can have aduration of less than 1 ns. For example, the first pulse of radiationcan have a duration of 300 ps or less, 100 ps or less, between 100-300ps, or between 10-100 ps.

The first pulse of radiation can be the first pre-pulse 6 (FIG. 1B) orthe pulse of radiation 317 (FIGS. 3A-3D). The altered droplet can be theintermediate target 51 (FIG. 1A) that is formed by irradiating thetarget material droplet 50 with the first pre-pulse 6. The targetmaterial droplet 50 can be a droplet of molten metal, such as tin or anyother material that emits EUV when converted to plasma. For example, thealtered droplet can be a disk of molten tin formed by striking thetarget material droplet 50 with the first pre-pulse 6. The force of theimpact of the first pre-pulse 6 can deform the droplet into a shape thatis closer to a disk that expands, after about 1-3 microseconds (μs),into a disk shaped piece of molten metal. In this example, the diskshaped piece can be considered the intermediate target 51. FIGS. 6C and8C show an exemplary intermediate target 613 that is disk shaped.

The altered droplet or intermediate target can take other geometricforms. For example, in implementations in which the first pulse ofradiation is less than 1 ns in duration, the altered droplet can have ashape that is formed from slicing a spheroid along a plane, such as ahemisphere like shape. FIG. 10C shows an exemplary intermediate target1014 that has a hemisphere shape. In the example shown in FIG. 10C, theintermediate target 1014 is a volume of particles instead of a diskshaped segment of molten tin.

A second pulse of radiation is directed toward the altered droplet toform an absorption material (420). The absorption material is the target55 that receives the amplified light beam and is converted to plasma (byionization due to the interaction of the amplified light beam with thetarget 55) that emits EUV light. The second pulse of radiation hasenergy sufficient to change a property of the altered droplet that isrelated to absorption of radiation. In other words, striking the altereddroplet formed in (420) with the second pulse of radiation changes theability of the altered droplet to absorb radiation, such as light.Further, the property related to absorption of radiation is changed suchthat the absorption material is able to absorb a higher portion ofincident radiation than the altered droplet.

The second pulse of radiation can have a duration of at least 1 ns andan energy of 1-10 mJ. For example, the second pulse of radiation canhave a duration of 10 ns and an energy of 5 mJ. The second pulse ofradiation can have a wavelength of 1.06 μm. The second pulse ofradiation can be the second pre-pulse 7 (FIG. 1B) or the pulse ofradiation 318 (FIGS. 3A-3D). Although the energy of the second pulse ofradiation can be lower and/or the pulse duration can be longer than apre-pulse applied directly to the target material droplet, theabsorption material (such as the target 55) has physical properties thatmake the target 55 favorable for generating EUV light.

In one example, the intermediate target 51 is a disk of molten tin that,as compared to the target material droplet 50, is thinner along adirection of propagation of an incident pulse of radiation. Thisintermediate target 51 is more easily broken into fragments of targetmaterial than the target material droplet 50, and a smaller amount ofenergy may be needed to fragment the intermediate target 51. In thisexample, the second pulse of radiation transforms the intermediatetarget 51 into a cloud of pieces of target material that, taken togetheror collectively, have a greater surface area of target material in thepath of an oncoming pulse of radiation as compared to the targetmaterial droplet 50. The greater surface area provides more targetmaterial for interaction with an amplified light beam and can lead toincreased ionization of the target material and therefore increased EUVlight generation. FIG. 7 shows an example of a second pulse of radiationthat transforms the intermediate target 51 into a target 55 thatincludes fragments of target material.

In another example, the intermediate target 51 is again a disk of moltentin that is thinner and wider than the target material droplet. In thisexample, the second pre-pulse irradiates the intermediate target 51 andgenerates a cloud of electrons and ions (a pre-plasma) close to thesurface of the intermediate target that receives the second pulse ofradiation. By creating the cloud of electrons and ions at the surface ofthe intermediate target 51, the second pulse of radiation alters theelectron density and/or the ion density of at least a portion of theintermediate target 51. FIG. 5 shows an example of a second pulse ofradiation that changes the electron density and/or ion density of atleast part of the modified droplet.

An amplified light beam is directed to the absorption material (430).The amplified light beam has energy sufficient to ionize and thereforeconvert target material in the absorption material (the intermediatetarget 51) into a plasma that emits EUV light. The amplified light beamcan be the amplified light beam 8 (FIG. 1B).

The target 55 and the waveform 5 discussed above provide examples. FIGS.5, 7, and 9 show representations of other exemplary waveforms 500, 700,and 900, respectively, for generating a target. FIGS. 6A-6E, 8A-8E, and10A-10E show energy of the waveforms 500, 700, and 900 being applied toa target material droplet.

Referring to FIG. 5, a plot of an example waveform 500 that can be usedto convert a target material droplet to a target that emits EUV light isshown. FIGS. 6A-6D show the waveform 500 transforming a target materialdroplet to the target that emits EUV light. The target of the example ofFIG. 5 and FIGS. 6A-6D is a flat disk of molten metal that has apre-plasma formed at a surface that faces an oncoming amplified lightbeam. The surface can face the oncoming amplified light beam if it ispointed towards the amplified light beam, even if the surface is nottransverse to the direction of propagation of the amplified light beam.

The waveform 500 shows a representation of a first pre-pulse 502, arepresentation of a second pre-pulse 504, and a representation of anamplified light beam 506. In this example, the first pre-pulse 502 has apulse duration 503 that is 20-70 ns, and an energy of 15-60 mJ. Forexample, the first pre-pulse 502 can have a wavelength of 1 μm or 10.6μm. In one example, the pulse duration 503 is 40 ns, and the energy is20 mJ. The second pre-pulse 504 can have a pulse duration 505 that is1-10 ns, an energy of 1-10 mJ, and a wavelength of 1.06 μm. In oneexample, the duration 505 of the second pre-pulse 503 is 10 ns, and theenergy of the second pre-pulse is 1 mJ.

The first pre-pulse 502 and the second pre-pulse 504 are separated intime by a delay time 508, with the second pre-pulse 504 occurring afterthe first pre-pulse 502. The delay time 508 is a time that is longenough to allow a target material droplet that is geometrically alteredthrough an interaction with the first pre-pulse 502 to expand to formthe intermediate target 51. The delay time 508 can be 1-3 microseconds(μs).

The second pre-pulse 504 and the amplified light beam 506 are separatedin time by a delay time 509, with the amplified light beam 506 occurringafter the second pre-pulse 504. The delay time 509 is long enough toallow the pre-plasma that the second pre-pulse 504 forms at the surfaceof the disk shaped target to expand. The delay time 509 can be between10-100 ns or between 1-200 nanoseconds (ns).

FIGS. 6A-6E show side views of a target material supply apparatus thatreleases target material droplets toward a target location 626 at fivedifferent times, t₁-t₅. The target location 626 is a location in achamber (such as a chamber 340) that receives the amplified light beam506 and is at the focus of the collecting optics 346 (FIG. 3A) or themirror 135 (FIG. 1A). FIG. 6A shows the earliest time, t₁, and the timeincreases from left to right, with FIG. 6E showing the latest time, t₅.A target material supply apparatus 620 releases a stream of dropletsthrough a nozzle 624. The stream of droplets includes target materialdroplets 611 and 610, with the target material droplet 610 beingreleased from the nozzle 624 before the target material droplet 610.FIGS. 6A-6E show the target material droplet 610 being transformed intoa target 614 that emits EUV light when struck by the amplified lightbeam 506.

Referring to FIG. 6A, the target material droplet 610 is struck by thefirst pre-pulse 502. As shown in FIG. 6B, the impact of the firstpre-pulse 502 geometrically deforms and spreads the target materialdroplet 610 into an elongated segment of target material 612. Theelongated segment 612 can have a shape that is disk-like and theelongated segment 612 can be molten target material. The elongatedsegment of target material 612 expands spatially as it travels towardthe target location 626. The elongated segment of target material 612expands for 1-3 μs (the delay time 508).

Referring to FIG. 6C, at time=t₃, which is 1-3 μs after the firstpre-pulse 502 strikes the target material droplet 610, the oblong shapedmaterial 612 has expanded into a disk shaped intermediate target 613 asit follows its trajectory toward the target location 626. Referring alsoto FIG. 6F, the intermediate target 613 has a width 632 and a thickness630. The thickness 630 of the intermediate target 613 is less than thewidth. In the example shown in FIGS. 6C and 6F, the width 632 is in the“x” direction and the thickness 630 is in the “y” direction, and thewidth 632 is along a direction that is transverse to the direction ofpropagation of the second pre-pulse 504. However, the intermediatetarget 613 can have other angular placements. For example, as shown inFIG. 6G, the intermediate target 613 can be angled 45° relative to thedirection of propagation of the second pre-pulse 504. Even when theintermediate target 613 is angled relative to the path of the secondpre-pulse 504, a thickness 631 of the intermediate target 613 measuredalong the direction of propagation of the pre-pulse 504 is less than thewidth of the intermediate target 613. As such, an oncoming light beam(such as the second pre-pulse 504) encounters less target area along adirection of propagation than along a plane that is perpendicular to thepath that the oncoming light beam would travel if it went directlythrough the target material droplet 610.

Referring also to FIG. 6D, the interaction between the second pre-pulse504 and the intermediate target 613 forms a target 614. The interactioncreates a pre-plasma 615 that is close to a bulk target material 616.The bulk target material 616 can be target material and can be moltenmetal. The pre-plasma 615 is allowed to expand over the delay time 509,and the expanded plasma 615 and the bulk target material 616 form thetarget 614. At time t₄, the target 614 arrives at the target location626.

In greater detail, the second pre-pulse 504 impinges on a surface of theintermediate target 613 and heats the surface to form the pre-plasma615. Because the intermediate target 613 is shaped like disk with thethin dimension presented to the pre-pulse 504, the pre-plasma 615 canutilize a higher portion of the target material in the bulk material616. After the pre-plasma has expanded for 1-200 ns the pre-plasma andthe bulk target material 616 are collectively called the target 614. Theamplified light beam 8 arrives at the target 614 before the pre-plasma615 blows off or dissipates. For example, the amplified light beam 8 canarrive 10-100 ns or 1-200 ns after the second pre-pulse 504 strikes theintermediate target 613. Because the pre-plasma 615 is present when theamplified light beam 506 arrives, the amplified light beam 506encounters the pre-plasma 615 prior to reaching the underlying bulktarget material 616. Compared to the underlying bulk target material616, the pre-plasma 615 is less reflective and absorbs the amplifiedlight beam 506 more readily. Thus, the presence of the pre-plasma 615allows a larger portion of the amplifying light beam 506 to be absorbed.

Further, in the absence of the pre-plasma 615, the amplified light beam506 impinges on the bulk target material 616 directly. In this instance,the amplified light beam 8 would encounter a metal surface and wouldmostly be reflected, with a small amount of the amplified light beam 8being absorbed to ablate the surface of the bulk target material 616 andform a pre-plasma cloud near the surface. The cloud can be formed 5-20ns after a pulse impinges on the surface. However, many pulses that haveenergy sufficient to convert the target material to plasma that emitsEUV light have a steep leading edge in the first 10-20 ns of the pulse.The amplified light beam 506 has a leading edge 510 (FIG. 5). Theintensity of the leading edge 510 (the portion of the pulse that reachesthe target surface over the first 10-20 ns of interaction between thepulse and the target) increases rapidly as a function of time, andincreases before the cloud of electrons and ions has had a chance toform and before the heating and evaporation process begins. Thus,without the pre-plasma 615, much of the energetic leading edge 510 ofthe amplified light beam 8 would be reflected and largely unused.However, the pre-plasma 615 absorbs a portion of the energy in theleading edge 510 and converts it to heat that ablates the bulk targetmaterial 616.

Referring to FIG. 6E, the amplified light beam 506 converts most ornearly all of the pre-plasma 615 and the bulk target material 616 intoEUV light 618.

Referring to FIG. 7, a plot of another exemplary waveform 700 that canbe used to convert a target material droplet to a target that emits EUVlight is shown. FIGS. 8A-8E show the waveform 700 transforming a targetmaterial droplet to the target that emits EUV light. The target of theexample of FIG. 7 and FIGS. 8A-8E is a collection of fragmented targetmaterial.

The waveform 700 shows a representation of a first pre-pulse 702, arepresentation of a second pre-pulse 704, and representation of a anamplified light beam 706. The first pre-pulse 702 has a pulse duration703 that is 20-70 ns, and an energy of 17-60 mJ. The first pre-pulse 702can have a wavelength of 1 μm or 10.6 μm. In one example, the pulseduration 703 is 40 ns, and the energy is 20 mJ. The second pre-pulse 704has a pulse duration 705 that is 1-10 ns and an energy of 1-10 mJ. Thesecond pre-pulse 704 has a wavelength of 1.06 μm. In one example, theduration 705 of the second pre-pulse 703 is 10 ns, and the energy of thesecond pre-pulse is 5 mJ. In another example, the duration 705 of thesecond pre-pulse 703 is 10 ns, and the energy of the second pre-pulse is10 mJ.

The first pre-pulse 702 and the second pre-pulse 704 are separated intime by a delay time 708, with the second pre-pulse 704 occurring afterthe first pre-pulse 702. The delay time 708 is a time that is longenough to allow a target material droplet that is geometrically deformedby the first pre-pulse 702 to expand to form an disk shaped intermediatetarget. The delay time 708 can be 1-3 microseconds (μs).

The second pre-pulse 704 and the amplified light beam 706 are separatedin time by a delay time 709, with the amplified light beam 706 occurringafter the second pre-pulse 704. The delay time 709 is long enough toallow the fragments that the second pre-pulse 704 forms to disperse toan optimal distance. The delay time 709 can be 100 nanoseconds (ns) to 1microsecond (μs).

Referring to FIGS. 8A-8E, five snap shots of the target material supplyapparatus 620 are shown, with time increasing from FIG. 8A on the leftto FIG. 8E on the right. FIGS. 8A-8C produce the disk-shapedintermediate target 613 as discussed with respect to FIGS. 6A-6C. FIG.8D shows the generation of the target 814. The target 814 is acollection of pieces or particles of target material that is formed byirradiating the intermediate target 613 with the second pre-pulse 704.The impact of the second pre-pulse 704 breaks the intermediate target613 into many fragments of target material, each of which is smallerthan the intermediate target 613.

Breaking the intermediate target 613 into the fragments provides moretarget material for the amplified light beam 706 because, collectively,the fragments present more surface area of target material forconversion to plasma. Moreover, because of the thinness of theintermediate target 613, the second pre-pulse 704 may be relatively lessenergetic and/or longer in duration than a pre-pulse capable oftransforming the target material droplet 610 into a collection offragments.

The target 814 arrives in the target location 626 and receives theamplified light beam. EUV light 818 is produced.

Referring to FIG. 9, a plot of another exemplary waveform 900 that canbe used to convert a target material droplet to a target that emits EUVlight is shown. FIGS. 10A-10E show the waveform 900 transforming atarget material droplet to the target that emits EUV light. The targetof the example of FIG. 9 and FIGS. 10A-10E is a pre-plasma that isformed close to a hemisphere shaped target.

The waveform 900 shows a representation of a first pre-pulse 902, arepresentation of a second pre-pulse 904, and a representation of anamplified light beam 906. The first pre-pulse 902 has a pulse duration903 that is less than 1 ns. For example, the first pre-pulse 902 canhave a wavelength of 1.06 μm, a pulse duration of 300 ps or less, and anenergy of 1 mJ-10 mJ. In another example, the first pre-pulse has aduration of 100 ps-300 ps, a wavelength of 1.06 μm, and an energy of 1mJ-10 mJ. In yet another example, the first pre-pulse 902 has a durationof 150 ps, a wavelength of 1.06 μm, and an energy of 5 mJ.

The second pre-pulse 904 has a pulse duration 905 that is 1-10 ns and anenergy of 1-10 mJ. The second pre-pulse 904 has a wavelength of 1.06 μm.In one example, the duration 905 of the second pre-pulse 903 is 10 ns,and the energy of the second pre-pulse is 5 mJ. In another example, theduration 905 of the second pre-pulse 903 is 10 ns, and the energy of thesecond pre-pulse is 10 mJ.

The first pre-pulse 902 and the second pre-pulse 904 are separated intime by a delay time 908, with the second pre-pulse 904 occurring afterthe first pre-pulse 902. The delay time 908 is a time that is longenough to allow a target material droplet that is geometrically deformedby the first pre-pulse 902 to expand to form a hemisphere shaped target.For example, the delay time 908 can be about 1000 ns. The delay time 909is long enough to allow the pre-plasma that the second pre-pulse 904forms at the surface of the hemisphere shaped target to expand. Thedelay time 909 can be 10-100 nanoseconds (ns) or 1-200 ns.

Referring to FIGS. 10A-10E, five snap shots of the target materialsupply apparatus 620 are shown, with time increasing from FIG. 10A onthe left to FIG. 10E on the right. The first pre-pulse 902 irradiatesthe target material droplet 610 to form a hemisphere shaped volume 1012.The hemisphere shaped volume 1012 is a mist or collection of particles1013 that are distributed throughout a hemisphere shaped space. Theparticles 1013 are distributed with a density distribution that isminimum at a surface 1002 that faces toward the second pre-pulse 904.The direction of increase of the density distribution contributes to anincreased amount of light being absorbed by the volume 1012 because mostof the light is absorbed by the volume 1012 before the light reaches aplane of high density that could reflect the light. The hemisphereshaped volume 1012 expands over the delay time 909 to form thehemisphere shaped intermediate target 1014.

The second pre-pulse 904 irradiates the hemisphere shaped intermediatetarget 1014 to generate a pre-plasma at an edge of the intermediatetarget 1014 and also converts at least some of the particles 1013 intothe pre-plasma. Because the particles 1013 are small, it is relativelyeasy to generate a pre-plasma from the particles 1013. The pre-plasmaexpands over the delay time 909 to form the target 1015. The target 1015includes a hemisphere shaped volume 1017 and a pre-plasma 1016. Theamplified light beam 906 irradiates the target 1015 to generate EUVlight. The pre-plasma 1016 provides a medium that absorbs the amplifiedlight beam 906 readily, thus, the pre-plasma 1016 can enhance andimprove the conversion of the amplified light beam into EUV light.

Other implementations are within the scope of the following claims. Forexample, the disk shaped intermediate target 613 can have a shape thatis similar to a disk or that includes an indentation in one of thesurfaces. Any of the waveforms 5, 500, 700, and 900 discussed above canhave more than two pre-pulses that interact with target material.

1. (canceled)
 2. An extreme ultraviolet light source comprising: anoptical source configured to produce a first pulse of radiation, asecond pulse of radiation, and an amplified light beam; a vacuum chamberthat receives a target material droplet at a first location in aninterior of the vacuum chamber, the target material droplet comprising atarget material that emits extreme ultraviolet (EUV) light whenconverted to plasma; a control system comprising machine-executableinstructions on a computer-readable medium, the control systemconfigured to control a timing circuit of the optical source to causethe optical source to: emit the first pulse of radiation, the firstpulse of radiation comprising an energy sufficient to transform thetarget material droplet into a geometric distribution of target materialthat occupies a larger volume than a volume occupied by the targetmaterial droplet, emit the second pulse of radiation, the second pulseof radiation comprising an energy sufficient to change an absorptioncharacteristic of the geometric distribution to form a modified targetthat absorbs a greater portion of incident radiation than the targetmaterial droplet or the geometric distribution, and emit the amplifiedlight beam, the amplified light beam comprising an energy sufficient toconvert at least some of the modified target into plasma that emits EUVlight; and an optical element between the source and the vacuum chamber,the optical element configured to: receive the first pulse of radiation,the second pulse of radiation, and the amplified light beam, and directthe first pulse of radiation, the second pulse of radiation, and theamplified light beam to, respectively, the first location in theinterior of the vacuum chamber, a second location in the interior of thevacuum chamber, and a third location in the interior of the vacuumchamber, the first, second, and third locations being differentlocations along a direction that is different than a direction ofpropagation of the first pulse, the second pulse, and the amplifiedlight beam in the vacuum chamber.
 3. The extreme ultraviolet lightsource of claim 2, wherein the first, second, and third locations in theinterior of the vacuum chamber are different locations along a directionthat is perpendicular to at least one of the direction of propagation ofthe first pulse, the second pulse, and the amplified light beam in thevacuum chamber.
 4. The extreme ultraviolet light source of claim 2,further comprising a target material delivery system coupled to thevacuum chamber, the target material delivery system configured toprovide the target material droplet to the first location in theinterior of the vacuum chamber.
 5. The extreme ultraviolet light sourceof claim 4, wherein the target material delivery system releases thetarget material onto a trajectory, the trajectory comprising the firstlocation, and the first, second, and third locations in the interior ofthe vacuum chamber are different locations along the trajectory.
 6. Theextreme ultraviolet light source of claim 2, wherein the sourcecomprises a first source and a second source, separate from the firstsource, the first source configured to produce the first pulse ofradiation and the second pulse of radiation, and the second sourceconfigured to produce the amplified light beam.
 7. The extremeultraviolet light source of claim 6, wherein the first pulse ofradiation is at a first wavelength, the second pulse of radiation is ata second wavelength, and the amplified light beam is at a thirdwavelength, and the first wavelength and the second wavelength comprise1.06 microns (μm), and the third wavelength comprises 10.6 μm.
 8. Theextreme ultraviolet light source of claim 6, wherein the second sourcecomprises a carbon dioxide (CO₂) laser.
 9. The extreme ultraviolet lightsource of claim 6, wherein the first source comprises two sources, andthe first pulse of radiation is produced by one of the two sources, andthe second pulse of radiation is produced by the other of the twosources.
 10. The extreme ultraviolet light source of claim 2, whereinthe source comprises a first source and a second source, separate fromthe first source, the first source configured to produce the first pulseof radiation and the amplified light beam, and the second sourceconfigured to produce the second pulse of radiation.
 11. The extremeultraviolet light source of claim 10, wherein the first pulse ofradiation is at a first wavelength, the second pulse of radiation is ata second wavelength, and the amplified light beam is at a thirdwavelength, and the first source comprises a CO₂ laser, the firstwavelength of first pulse of radiation is associated with a firstemission line of the CO₂ laser, and the third wavelength of theamplified light beam is associated with a second emission line of theCO₂ laser.
 12. The extreme ultraviolet light source of claim 11, whereinthe first wavelength comprises 10.26 μm, and the third wavelengthcomprises 10.59 μm.
 13. The extreme ultraviolet light source of claim 2,further comprising at least one optical amplifier between the source andthe vacuum chamber, and wherein the first pulse of radiation, the secondpulse of radiation, and the amplified light beam pass through the atleast one amplifier.
 14. The extreme ultraviolet light source of claim2, wherein the optical element is configured to direct the first pulseof radiation, the second pulse of radiation, and amplified light beamtoward, respectively, the first, second, and third locations in thevacuum chamber by angularly disbursing at least two of the first pulseof radiation, the second pulse of radiation, and the amplified lightbeam relative to a surface of the optical element.
 15. The extremeultraviolet light source of claim 2, wherein the optical elementcomprises a fiber optic delivery system.
 16. A method comprising:directing a first pulse of radiation toward a first location in a vacuumchamber of an extreme ultraviolet (EUV) source, the first locationreceiving a target material droplet comprising target material thatemits EUV light when converted to plasma, and the first pulse ofradiation comprising an energy sufficient to transform the targetmaterial droplet into a geometric distribution of target material, thegeometric distribution of target material occupying a larger volume thana volume occupied by the target material droplet; directing a secondpulse of radiation toward a second location in the vacuum chamber toform a modified target from the geometric distribution, the secondlocation being a different location than the first location; anddirecting an amplified light beam toward a third location in the vacuumchamber, the third location being a different location than the firstlocation and the second location, and the amplified light beamcomprising an energy sufficient to convert at least some of the modifiedtarget to plasma that emits EUV light, wherein the modified targetabsorbs a greater portion of incident light than the geometricdistribution.
 17. The method of claim 16, wherein the modified targetcomprises a contiguous segment of target material.
 18. The method ofclaim 17, wherein the contiguous segment of target material comprises acontinuous segment of molten tin.
 19. The method of claim 16, whereinthe modified target comprises a pre-plasma adjacent to a surface ofcontinuous segment of target material.
 20. The method of claim 16,wherein the modified target comprises a collection of pieces of targetmaterial.
 21. The method of claim 20, wherein the collection of piecesof target material are in a hemisphere-shaped volume.
 22. The method ofclaim 21, wherein the hemisphere-shaped volume comprises a roundedportion that extends along a direction of propagation of the amplifiedlight beam.
 23. The method of claim 16, further comprising directing EUVlight emitted by the plasma to a lithography tool.
 24. Aphotolithography system comprising: a lithography tool configured toprocess wafers; and an extreme ultraviolet light source comprising: anoptical source configured to produce a first pulse of radiation, asecond pulse of radiation, and an amplified light beam; a vacuum chamberthat receives a target material droplet at a first location in aninterior of the vacuum chamber, the target material droplet comprising atarget material that emits extreme ultraviolet (EUV) light whenconverted to plasma; an EUV collecting optic in the vacuum chamberconfigured to direct EUV light emitted by the plasma to the lithographytool; a control system comprising machine-executable instructions on acomputer-readable medium, the control system configured to control atiming circuit of the optical source to cause the optical source to:emit the first pulse of radiation, the first pulse of radiationcomprising an energy sufficient to transform the target material dropletinto a geometric distribution of target material that occupies a largervolume than a volume occupied by the target material droplet, emit thesecond pulse of radiation, the second pulse of radiation comprising anenergy sufficient to change an absorption characteristic of thegeometric distribution to form a modified target that absorbs a greaterportion of light than the target material droplet or the geometricdistribution, and emit the amplified light beam, the amplified lightbeam comprising an energy sufficient to convert at least some of themodified target into plasma that emits EUV light; and an optical elementbetween the source and the vacuum chamber, the optical elementconfigured to: receive the first pulse of radiation, the second pulse ofradiation, and the amplified light beam, and direct the first pulse ofradiation, the second pulse of radiation, and the amplified light beamto, respectively, the first location in the interior of the vacuumchamber, a second location in the interior of the vacuum chamber, and athird location in the interior of the vacuum chamber, the first, second,and third locations being different locations along a direction that isdifferent than a direction of propagation of the first pulse, the secondpulse, and the amplified light beam in the vacuum chamber.